(1) Field of the Invention
The invention relates to a method of photolithographic etching of metal lines, and more particularly, to a method of photolithographic etching of sub-quarter micron metal lines without undercutting in the manufacture of integrated circuits.
(2) Description of the Prior Art
It is desired for metal lines to have a vertical profile. This is not always easy to achieve, especially for sub-quarter micron metal lines. FIG. 1 illustrates in cross-sectional representation a partially completed integrated circuit device. Semiconductor substrate 40 contains semiconductor devices structures, not shown. A metal line stack is shown on the substrate. Barrier metal layer 44 (for example, titanium/titanium nitride) is on the bottom of the stack. The metal layer 46, such as AlCu, overlies the barrier layer. An antireflective coating (ARC) 48 is at the top of the stack. Photoresist mask 50 is used in etching the metal line.
In order to improve lithographic resolution in the formation of sub-quarter micron metal lines, the photoresist layer must become thinner. However, if the photoresist layer is too thin, the top corner 52 of the metal line may be damaged during the etching of a high aspect ratio metal line. Therefore, it is necessary to use a hard mask during metal etching in sub-quarter micron technology to protect the metal top corners without using a thicker photoresist mask.
FIG. 2 illustrates a partially completed integrated circuit device as in FIG. 1 except that an oxide hard mask 49 has been formed overlying the ARC layer. Conventionally, the etchant gases are BCl.sub.3, Cl.sub.2, and N.sub.2. These gases have been found to be insufficient in producing a passivation layer on the sidewalls of the AlCu lines which would prevent Cl.sub.2 erosion and undercutting during etching. The undercutting 53 is illustrated in FIG. 2.
One proposed solution to the undercutting problem is the use of SF.sub.6 gas in the overetch step to react with titanium from the barrier layer and AlCu to form AlF.sub.x or TiF.sub.x as a passivation layer. However, SF.sub.6 is also the etching gas used in the tungsten etchback process. Tungsten plugs are likely to underlie the metal lines in the substrate, as illustrated in FIG. 3. Especially at the endcap of the metal line, as shown in FIG. 3, the SF.sub.6 etchant gas will damage the integrity of the tungsten plug 54.
U.S. Pat. No. 5,460,693 to Moslehi uses a fluorinated layer as a mask instead of photoresist. An oxide is formed on the unexposed areas of the fluorinated layer to form a hard oxide mask. U.S. Pat. No. 5,591,676 to Hughes et al teaches etching a fluorinated polymer using a hard oxide mask. U.S. Pat. No. 5,369,053 to Fang teaches using an oxide hard mask under a thin photoresist layer and etching the underlying metal using both a fluorine-based etchant and a chlorine-based etchant. U.S. Pat. No. 5,350,484 to Gardner et al teaches a method of implanting ions into an exposed area of metal and then selectively removing the implanted metal.